Hundreds of millions of tires are scrapped each year due to wear or damage. Presently, the United States alone generates approximately 270,000,000 scrap tires per year. About twenty-six percent of scrap tires each year end up in landfills and stockpiles, and in only about ten percent of the time the rubber is recovered and re-used. In total, scrap tire inventories are estimated to be between 2 and 3 billion tires.
Efforts have been made to find uses for scrap tires to minimize the inventories. Some of the uses for the scrap whole tires include artificial fishing reefs, floating breakwaters, and impact absorbers around highway and bridge abutments. Whole or cut up portions of scrap tires are also used in playgrounds, flower planters, shoe soles and dock bumpers. Whole tires and shredded tire chips may also be burned and used as fuel. Reclaimed tire rubber or crumb rubber can also be used to make low value rubber products such as floor mats and in asphalt for roadways.
Currently, only low amounts of recycled rubber such as tire rubber, in the form of vulcanized or crosslinked crumb rubber particles, are used in tires (typically less than about 10% by weight of the rubber in the tire). The reason for the low level of usage is that the addition of crumb rubber in virgin rubber compounds lowers important functional properties in the tire such as high strain modulus and tensile strength.
Therefore, the development of a crumb rubber that can be used in levels above 10% in a virgin rubber tire compound without significantly degrading the properties of a tire would be desirable.
An excellent summary of the status of tire and rubber recycling, rubber reclaiming technologies and processes, methods to obtain “crumb rubber” and technologies to treat or modify crumb rubber to make it more usable in formations is given in the article “Rubber Recycling”, Rubber Chemistry and Technology, Vol. 75, pages 429-469, by Marvin Myhre and Duncan A. MacKillop.
More recently, recycling efforts have focused on treating or modifying crumb rubber particles to make it more dispersible and cure compatible with other ingredients in formulations. This is the direction of the present invention.
The invention comprises the modification of crumb rubber through the reaction between crumb rubber and a Silicon hydride to form carbon-silicon bonds. The crumb rubber so modified can be used at higher levels in virgin rubber formulations without significantly degrading important properties.
The chemistry of reacting silicon hydride with >C═C< unsaturation (or carbon-carbon double bonds) is known. U.S. Pat. Nos. 2,637,738; 2,823,218; and 3,159,662 disclose the reaction of silicon hydrides having an active hydrogen site with unsaturated molecules and compounds to produce new compounds having a carbon-silicon bond. The unsaturated compound can be a low molecular weight compound or a polymer such as an uncured or uncrosslinked rubber. The reaction is typically catalyzed using a platinum catalyst such as commercial Speir's catalyst (chloroplatinic acid) or Karsted's catalyst (Pt zero complex with divinyltetramethyl disiloxane). This is done because the uncatalyzed reaction is described to be slow, unless conducted at relatively high temperatures, and inefficient with low yields of desired product and production of many unwanted by-products.
A platinum-catalyzed hydrosilation crosslinking reaction is described in U.S. Pat. No. 5,672,660. Here, an uncured diene-containing elastomer is reacted with a Silicon hydride in the presence of a thermoplastic resin and a platinum catalyst to prepare a thermoplastic elastomer.
More recently, US Publication No. US 2004/0030053 A1 (Feb. 12, 2004) shows the preparation of a surface-modified rubber useful as a recycled material. This material is prepared by the reaction of a waste (crumb) rubber with a silane-coupling agent having a mercapto group or an S—S bond. This reaction forms a sulfur-carbon (S—C) bond and not a silicon-carbon (Si—C) bond, as described in Volume 69, page 325 Rubber Chemistry and Technology, 1996 by Siegfried Wolff entitled Chemical Aspects of Rubber Reinforcement by fillers (see pages 328-344).